FIG. 1 shows a system of the prior art fitted with transponders of the aforecited type. This system includes a reader, fitted with an RF (radio-frequency) communication antenna 3 and then includes at least one transponder 4 that is also fitted with an RF communication antenna 5. Transponder 4 includes an envelope detector 8 for modulated signals received by antenna 5, an analogue RF input-output circuit 10 and a logic part (not shown in FIG. 1). The RF analogue front part 10 essentially defines two branches or paths 12 and 14 between the antenna and the logic part of the transponder. Input path 12 passes through an input amplifier 16 and then a demodulation unit 18, which supplies a logic signal to the logic part of the transponder. Path 14 between the logic part and the antenna forms the transponder response means. This path 14 thus passes through a modulation unit 20 and then through an output amplifier 22.
Transponder 4 is fitted with wake up means that are associated with input amplifier 16. At least this input amplifier is continually active and thus participates in standby mode. When it receives a signal of higher amplitude than a predetermined reference value, the input amplifier activates the top levels and/or parts of the transponder that are inactive in standby mode.
In general, communications systems of the type described in FIG. 1 communicate at a relatively high frequency, the frequency ranges used in active transponder systems being higher and higher in the frequency range, particularly in the megahertz and gigahertz domain. Thus, input amplifier 16 is a high frequency amplifier, which generally has a relatively broad bandwidth due to the fact that it operates at a high frequency.
The arrangement of wake up means in active transponder 4 is used to reduce its electrical power consumption so as to increase its longevity. Thus, the part of the transponder that has to be continually active is limited, to ensure that the transponder is activated upon reception of an interrogation signal from a reader. In the case of the prior art, at least high frequency input amplifier 16 constitutes the continuously powered part.
The system of the prior art described above has various drawbacks, related to the electric power consumption of transponder 4 when the latter is in standby mode. We can identify at least two causes of an increase in the electric power consumption of the transponder in standby mode. Firstly, a high frequency input amplifier has relatively large electric power consumption and, secondly, these high frequency amplifiers generally have quite a broad bandwidth, so that they are capable of amplifying interference signals received by the transponder antenna at frequencies close to those provided for the system concerned. Systems available on the market generally operate within given frequency bands. Thus, different systems may operate at a high frequency in a same, relatively small frequency band in relative value. Input amplifier 16 reacts to the signals received within its bandwidth, which thus includes frequencies belonging to other systems. As a result, interference signals can frequently waken the transponder. This obviously increases the electric power consumption of the transponder, which is woken inadvertently but returns to standby mode again when it has detected that the received signal was not intended for it.